The Metencephalic Basis of the Conditioned Nictitating Membrane Response

Summary

The classically conditioned nictitating membrane response of the rabbit has evolved into a useful model system for neurobiological studies of associative learning. Converging methodologies from behavioral, physiological, and anatomical approaches indicate key roles in conditioning for the limbic system, especially the hippo-campus, and the brain stem. This article presents evidence that the metencephalon contains neural elements essential for generation and performance of conditioned responses in this preparation.

Recordings of multiple-unit activity from chronically implanted microelectrodes during conditioning in alert animals suggest that the dorsolateral pons at the level of the trigeminal nerve contains populations of neurons which are a substrate of the conditioned response. Lesions in this region virtually abolish conditioned responding while leaving the unconditioned response to eye shock intact. This observation suggests that components of the reflex arc of the unconditioned response, including rostral elements of the sensory trigeminal system and of the accessory abducens nucleus,are not crucial for learning. Instead, associative convergence between conditioned and unconditioned stimuli appears to occur in an adjacent interneuron system of the supratrigeminal reticular formation.